Purification of Aromatic Feedstock

ABSTRACT

The invention is a process for removing impurities from an aromatics stream and apparatus for the practice thereof, whereby trace olefins and dienes are removed from aromatic plant feedstocks using a reactor design that enables the product to be backmixed with the feedstock and that enables a feed/effluent heat exchanger.

PRIORITY CLAIM

This application claims the benefit of U.S. Provisional Application No.61/377,322, filed Aug. 26, 2010, the entirety of which is incorporatedby reference.

FIELD OF THE INVENTION

The invention relates to removing trace olefins and dienes from aromaticfeedstocks.

BACKGROUND OF THE INVENTION

In petroleum processing, aromatic streams are derived from processessuch as naphtha reforming and thermal cracking (pyrolysis). Thesearomatic streams also contain undesirable hydrocarbon contaminantsincluding mono-olefins, dienes, styrenes and heavy aromatic compoundssuch as anthracenes.

The aromatic streams are used as feedstocks in various subsequentpetrochemical processes. In certain of these processes, such aspara-xylene production, e.g., from an aromatic stream containingbenzene, toluene and xylenes (BTX) or toluene disproportionation,hydrocarbon contaminants cause undesirable side reactions. Therefore thehydrocarbon contaminants must be removed before subsequent processing ofthe aromatic streams.

Moreover, the shift from high-pressure semiregenerative reformers tolow-pressure moving bed reformers results in a substantial increase incontaminants in the reformate derived streams. This in turn results in agreater need for more efficient and less expensive methods for removalof hydrocarbon contaminants from the aromatic streams.

Undesirable hydrocarbon contaminants containing olefinic bonds arequantified by the Bromine Index (BI). Undesirable olefins, includingboth dienes and mono-olefins, have typically been concurrently removedfrom aromatic streams such as BTX by contacting the aromatic stream withacid-treated clay. Other materials, e.g., zeolites, have also been usedfor this purpose. Clay is an amorphous naturally-occurring material,while zeolites used for this purpose generally are synthesized and aretherefore more expensive. Both clay and zeolites have limited lifetimesin aromatics treatment services. Although clay is much cheaper per sethan zeolites, under certain circumstances the choice of zeolites canprovide improved performance, making the selection of zeolites anoverall better economic choice.

Improvement in using zeolites to remove contaminants from aromatic feedshave been described in numerous patents, such as U.S. Pat. Nos.6,368,496; 6,500,996; 6,781,023; 7,214,840; 7,517,824; 7,731,839;7,744,750; U.S. Patent Application Publications 2007-0112239 and2008-0128329; and U.S. Provisional Application 61/240,424.

Olefin removal devices in use today, designed for use with clay, do notuse a feed distribution system. This is because feed distribution doesnot improve cycle length, as clay beds age in a band-wise fashion.Accordingly, a feed distribution system would just be an additional costwithout any benefit. Furthermore, clay can be completely exhausted at areaction temperature of 205° C. Reactor design heretofore typicallyprovides for a maximum temperature of about 210° C. Increasing thereactor temperature enables only small gains in run length. Achieving ahigher temperature requires use of hotter steam in the feed heater andaddition of a feed effluent heat exchanger since the product is too hotfor optimal integration with aromatics distillation towers.

The present inventor has discovered a process and apparatus for thepractice thereof, whereby trace olefins and dienes are removed fromaromatic plant feedstocks using a reactor design that enables theproduct to be backmixed with the feedstock and that has a feed/effluentheat exchanger.

SUMMARY OF THE INVENTION

The invention is directed to a process and apparatus for the practicethereof, whereby trace olefins and dienes are removed from aromaticplant feedstocks by contact of the feedstock with a catalyst comprisingat least one zeolite, using a reactor design that in embodiments enablesthe product to be backmixed with the feedstock and that in embodimentshas a feed/effluent heat exchanger. In embodiments, the design uses afeed distributor which in embodiments prevents channeling. While each ofthese embodiments may be achieved separately, it is preferred that atleast two and still more preferably all three embodiments are present inthe apparatus.

In embodiments, the design enables the feed to be heated to hightemperatures, such as 250° C., significantly higher than the 210° C.maximum feed temperature used in today's reactors, and still provideeffective removal.

The ability to recycle the reactor product provides two unexpectedbenefits. Recirculation of product without addition of fresh feedprovides an unexpected means of rejuvenating the zeolite catalystsystem. Recirculation of product while processing fresh feedunexpectedly increases cycle length. The inventor speculates that thebenefit is achieved by evenly distributing the reaction workload fromthe top to the bottom of the bed.

It is an object of the invention to provide increased serviceable lifeto a aromatic feed purification system comprising one or more zeolites.

These and other objects, features, and advantages will become apparentas reference is made to the following detailed description, preferredembodiments, examples, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing an apparatus according to an embodiment ofthe invention.

FIG. 2 is a plot of catalyst performance over time on stream for anembodiment of the process according to the invention.

DETAILED DESCRIPTION

According to the invention, there is a process for removing impuritiesfrom an aromatics stream and apparatus for the practice thereof, wherebytrace olefins and dienes are removed from aromatic plant feedstocks(also known in the art as reduction of Bromine Index or BI of saidfeedstock) by contact with at least one zeolite in a reactor thatenables the product to be backmixed with the feedstock and that providesa feed/effluent heat exchanger.

In the present invention, a feed/effluent heat exchanger is added toprevent the product from upsetting the distillation column when theproduct temperature exceeds 205° C. Current clay treater reactor design,modified by adding zeolite or replacing clay with zeolite, has a feedpre-heater capable of heating the feed generally to a maximum of 205°C., although in some cases it may operate as high as 220° C. The productis passed directly into a distillation column. All the heat in theproduct is used during the distillation. The product is depressured whendistilled. Only minimal flashing occurs under ordinary operatingconditions of existing clay treater reactors retrofitted with catalystscomprising zeolite. The present inventor has thus discovered a simpleand effect way of more effectively utilizing the benefits of a catalystcomprising zeolite and exceeding the prior art reactor temperature andoperating even as high as 300° C.

In addition, current clay treater design has no feed distributor. Thefeed enters the vessel through a large pipe. This is sufficient whenusing clay treaters because the clay undergoes band wise aging. Bandwise aging means that when the unit is restarted with a fresh clay, thetop of the clay bed reduces contaminants below the specification and therest of the clay bed is not exposed to these contaminants. After a weekon stream, the top of the clay bed is nearly completely inactive, havingfilled with coke, and a small section of the clay bed between the topand the bottom is removing the contaminants. The zone of clay removingthe contaminants as required moves from the inlet toward the outlet asthe run progresses. In other words, conversion activity is limited to a“band” which moves down the bed with time—hence band-wise aging. Incontrast, with catalyst comprising zeolite, one of the benefits overclay is that the entire bed is utilized because zeolite does not age inband-wise fashion. Accordingly there is benefit to be gained by havingfeed distributors.

The fluid distributor may take a number of forms, such as, for example,screens, grids, perforated plates, and the like. The fluid distributionserves several objectives. It is intended to distribute the fluidspassing downwardly through the reactor evenly across the horizontalplane of the catalyst layer, serves to insure the break-up of any gasbubbles and the prevention of jet action on the catalyst bed, and itinsures optimal mixing of the fluids, i.e., recycle and fresh feed. Suchfeed distribution systems are per se known, such as U.S. Pat. Nos.6,554,994 and 6,063,263. As used herein, then, the expression“substantially evenly distributed” is intended to mean that the fluid isdistributed over the top of the catalyst bed so as to substantiallyachieve the above objective, and the adjective “substantially” should betaken to mean that one of ordinary skill in the art, having the presentdisclosure before them, would be able to engineer a distribution deviceso to achieve these objectives.

The preferred catalyst for treatment to substantially removemono-olefins is selected from the MWW family of zeolites, which are perse well known. More preferably the catalyst is MCM-22, MCM-36, MCM-49,MCM-56, EMM-10, and mixtures thereof. Such catalysts have been describedin numerous patents and publications, such as U.S. Pat. No. 4,954,325,U.S. Pat No. 5,229,341, U.S. Pat. No. 5,236,575, and U.S. Pat. No.5,362,697.

After treatment to remove mono-olefins the resultant stream is distilledto obtain the product, which is preferably an overhead stream comprisingC8 product.

Current clay treater designs do not recycle effluent back through thereactor because there is no particular benefit to be achieved. Incontrast, recycling product using a catalyst comprising zeolite ishighly beneficial. However, recycling olefin-free product rejuvenatesthe catalyst comprising zeolite (illustrated in the experimental sectionbelow).

In embodiments, the catalyst is also mixed with a clay. In thealternative, the clay and zeolite could be separated from each other,such as in the same reactor or in different reactors, and also the claymay be upstream or downstream of the zeolite. Any clay suitable forprocessing hydrocarbons can be used, preferably Engelhard F-24 clay,Filtrol 24, Filtrol 25, and Filtrol 62, Attapulgus clay or Tonsil clay,with Engelhard F-24 clay being the most preferred. The catalyst maycomprise a mixture of clay and zeolite having an outer layer of mostlyclay or an outer layer of mostly zeolite or an intimate mixture of clayand zeolite, and in embodiments the proportion of zeolite:clay may rangefrom about 5:95 to about 95:5 or 10:90 to 90:10, with a preferred rangebeing from 30:70 to 50:50. In some embodiments no clay is present. Theratios as used herein are mass ratios unless otherwise indicated.

In embodiments, the zeolite is a regenerated or rejuvenated zeolite.Such zeolites are known in the prior art per se and are discussed, forinstance, in U.S. Pat. Nos. 6,900,151 and 7,517,824, and U.S. PatentPublication 2008-0029437. It may also be fresh zeolite or a mixture offresh, regenerated, or rejuvenated in all possible proportions. One ofordinary skill in the art in possession of the present disclosure candetermine the appropriate proportions for a particular need withoutundue experimentation. As used herein, the term “regenerated catalyst”means any catalyst that has gone through at least one regenerativeprocess, such as an oxidative process sufficient to regenerate thecatalyst, including but not limited to traditional coke burn regen. Theterm “rejuvenated catalyst” means any catalyst that has gone through atleast one rejuvenation process, including but not limited to an inertreductive agent under conditions sufficient to rejuvenate the catalyst.In an embodiment, the present invention includes a step of contact ofsaid catalyst bed with a sufficient amount of recycle and for asufficient time to rejuvenate and/or regenerate said catalyst. In thismanner, the BI reduction capacity of the catalyst is restored to apredetermined level.

The present invention may be better understood by reference to thefollowing examples and accompanying figures, which are exemplary of thepresent invention and should not be taken as limiting thereof.

In FIG. 1, the aromatic feedstream 1 to be purified is first passedthrough an optional heat exchanger 2 where the feed takes heat frompurified stream 9 as it passes from the zeolite beds 7 and 8 through tostill 14. The heat exchanger 2 may also have a supplemental source ofheat, such as steam. The heat exchanger 2 (as well as heat exchanger 5,discussed below) may be of the type per se known in the art. Thefeedstream 3 heat-exchanged with 9 then may be heated by steam 5, by wayof example, provided to heat exchanger 4 and passed at junction 6 to thezeolite bed reactors 7 and 8, the reactors having distributors 27 and28, respectively. All or a portion of the purified aromatics stream maythen be sent via conduit 9 to still 14 and/or all or a portion of thesame may be recycled, via conduits 10 and 11 with the aid of recyclepump 12 and then via conduit 13 to be distributed between the reactors 7and 8. The nature of the overheads 15 and bottoms 16, of course, variesdepending on the feed and operating conditions of the system andparticularly the operating conditions of the still. In the case of aheavy reformate feed 1 the overheads 15 would primarily be xylenes andthe bottoms product 16 would be C9+ aromatics. In the case of a benzeneextract feed 1 the overheads 15 would be primarily benzene and thebottoms product 16 would be C7+ aromatics.

Good distribution of the feed is needed to make the best use of thezeolite catalyst. When recycle operation is used, the distributors playan important role ensuring that the fresh feed (stream 1) is well mixedwith recycle stream 13. There are many efficient distributor designsknown to those skilled in the art. One example is a “shower head” designthat provides an even spray of liquid feed across the top of thecatalyst bed. Other common designs use perforated trays, or a series ofpipes with uniform sized holes (similar to a common lawn sprinkler).

The following are experiments intended to be representative of thepresent invention and not limiting thereof.

EXAMPLE 1

The reaction of benzene with ethylene was used to model the reaction ofbenzene, toluene, and xylenes with C6+ iso-olefins that occur inaromatics plant treatment units. Liquid feedstocks were prepared frombenzene, 2-methyl-pentane, 2,3 dimethylbutane, and methylcyclopentane.The feedstock compositions are provided in Table 1, below. Thesefeedstocks were blended with ethylene at mole ratios of ethylene tobenzene of 0.5:1 and 1:1. The blended feedstock was processed at 2-10WHSV, 250 psig, >98% ethylene conversion, and 350-400° F. (about177-204° C.). The catalyst was an MCM-22 family molecular sieve. Theexperiments were conducted in a standard laboratory fixed bed reactor.The liquid feed was delivered from a 500 cc syringe pump. The reactorwas constructed from a two foot long section of ⅜″ stainless steel tube.The reaction zone was heated with a three zone electrical furnace.

TABLE 1 Feed Compositions Model Isoparaffin 2,3-Dimethylpentane3-Methylpentane Nominal Composition, wt % 3-Methylpentane — 49.5Methylcyclopentane 1.0 1.0 Benzene 49.5 49.5 2,3-Dimethylpentane 49.5 —Composition wt % 2-Methylpentane — 0.047 3-Methylpentane — 43.791N-Hexane 0.008 0.055 Methylcyclopentane 0.923 0.939 2,2-Dimethylpentane0.079 — Benzene 51.718 55.163 3,3-Dimethylpentane 0.020 — Cyclohexane0.012 — 2,3-Dimethylpentane 47.021 — 3-Methylhexane 0.183 — Toluene —0.006 Other C₇ 0.037 — Total 100.001 100.001 EB Impurities*, ppmw Range= 0 vs Range = 4 21 20 Internal Standard 34, 30 — *All non-EB compoundseluting between toluene and cumene on a boiling point column

Catalyst performance vs. time on stream is plotted in FIG. 2. Thecatalyst aged steadily during the first 24 DOS (days-on-stream) asevidenced by rising levels of impurities between days 11 and 24 (soliddiamonds read off the left y-axis) and a steep fall off in ethyleneconversion after 24 DOS (open diamonds read off the right y-axis). Atthis time, ethylene was removed from the feedstock while remaining atconstant temperature, pressure, and benzene flowrate for 12 hrs. This12-hour period is represented on the graph by the vertical dashed linejust past 25 days on stream. Then the initial experiment was resumed.Surprisingly, this procedure restored the catalyst to fresh activity andselectivity.

The example demonstrates that olefin-free benzene feedstock rejuvenatesMCM-22 catalyst used for aromatics alkylation in the liquid phase at205° C.

The process of the invention provides a pure stream of aromatics as theproduct. In the reactor, olefin and diene contaminants react witharomatics to form larger aromatic molecules. Example one teaches thatcirculation of the olefin free stream at reaction temperature andpressure for several days rejuvenates MCM-22 catalyst. The rejuvenatedMCM-22 catalyst can then be restreamed with olefin and diene containingfeedstock. This procedure is much less expensive than unloading,regenerating, and reloading. After several rejuvenations the catalystwill need to be removed for standard regeneration.

COMPARATIVE EXAMPLE 2

A xylenes plant heavy reformate feedstock with a BI of 1000 is treatedover an MCM-22 catalyst in the reactor of the invention at 2 LHSV, 300psig, and 205° C. The BI of the xylenes produced from the reactoreffluent is monitored with time on stream. After 200 days, the BI of thexylenes reaches the end of cycle specification of 20.

EXAMPLE 3

The procedure of Example 2 is repeated except the unit is run with 3volumes of recycle for each volume of fresh feedstock. After 300 days,the BI of the xylenes approaches 20. At this point the recycle isstopped. The xylenes BI drops below 10 because the contaminant olefinsnow have a four times longer residence time. After 400 days, the BI ofthe xylenes reaches the end of cycle specification of 20. The examplesurprisingly doubles the catalyst cycle length.

While the illustrative embodiments of the invention have been describedwith particularity, it will be understood that various othermodifications will be apparent to and can be readily made by thoseskilled in the art without departing from the spirit and scope of theinvention.

All patents and patent applications, test procedures (such as ASTMmethods, UL methods, and the like), and other documents cited herein arefully incorporated by reference to the extent such disclosure is notinconsistent with this invention and for all jurisdictions in which suchincorporation is permitted. Trade names used herein are indicated by a ™symbol or ® symbol, indicating that the names may be protected bycertain trademark rights, e.g., they may be registered trademarks invarious jurisdictions. When numerical lower limits and numerical upperlimits are listed herein, ranges from any lower limit to any upper limitare contemplated.

What is claimed is:
 1. In an apparatus for the purification of anaromatic feedstock by contact of said feedstock with a catalyst bedcomprising at least one zeolite, whereby trace olefins and dienes areremoved from said aromatic feedstock to produce a reactor effluent,optionally wherein said effluent is subsequently distilled to produce asubstantially pure product, the improvement comprising at least one ofthe following two: (i) a feedstock distributor upstream of said catalystbed, whereby the feedstock is substantially evenly distributed over thetop of said catalyst bed, and (ii) wherein said reactor effluent and/orsaid substantially pure product are fluidly connected with said catalystbed to provide recycle; and optionally wherein a feedstock/effluent heatexchanger is provided, whereby heat from the reactor effluent isexchanged with said feedstock.
 2. The apparatus according to claim 1,wherein said at least one zeolite is selected from MWW structurezeolites.
 3. The apparatus according to claim 2, wherein said at leastone zeolite is selected from MCM-22 structure zeolites.
 4. The apparatusaccording to claim 1, wherein said catalyst bed further includes a clay.5. A process for the purification of an aromatic feedstock by contact ofsaid feedstock with a catalyst bed comprising at least one zeolite toproduce a reactor effluent having a lower BI index when compared withsaid feedstock, and then distilling said reactor effluent to provide anoverhead consisting essentially of substantially pure aromatichydrocarbon product, the improvement comprising at least one of thefollowing two: (i) a feedstock distributor upstream of said catalystbed, whereby the feedstock is substantially evenly distributed over thetop of said catalyst bed, and (ii) wherein said reactor effluent and/orsaid substantially pure product are fluidly connected with said catalystbed to provide recycle; and optionally wherein a feedstock/effluent heatexchanger is provided, whereby heat from the reactor effluent isexchanged with said feedstock.
 6. The process according to claim 5,wherein said at least one zeolite is selected from MWW structurezeolites.
 7. The process according to claim 6, wherein said at least onezeolite is selected from MCM-22 structure zeolites.
 8. The processaccording to claim 5, wherein said bed further includes a clay.
 9. Theprocess according to claim 5, including a step of contact of saidcatalyst bed with a sufficient amount of recycle and for a sufficienttime to rejuvenate said catalyst bed.
 10. The process according to claim5, wherein said contact occurs with said feedstream at a temperature ofgreater than 220° C.
 11. The process according to claim 5, wherein saidcontact occurs with said feedstream at a temperature of greater than240° C.
 12. The process according to claim 5, wherein said contactoccurs with said feedstream at a temperature of greater than 260° C. 13.The process according to claim 5, wherein said contact occurs with saidfeedstream at a temperature of greater than 280° C.